The invention is directed to a DC voltage converter.
PCT/EP91/00724 discloses such a voltage converter. The known voltage converter is a matter of a clocked voltage converter with current limitation that contains a first electronic switch in the main circuit and a precision resistor in series therewith. A second electronic switch with the assistance of which the current-proportional test voltage is evaluated is located between the precision resistor and a capacitor having a discharge circuit. A control circuit that controls the first electronic switch for the purpose of a limitation of the pulse current flowing in the main circuit as soon as the capacitor voltage lies above a predetermined limit is connected to the capacitor. The second electronic switch, which is otherwise closed, is switched into the conductive condition during the turn-on phase of the first electronic switch. The second electronic switch is formed by the source-drain path of a field effect transistor. The source-drain path of the field effect transistor is directed such that the inverse diode of the field effect transistor is polarized in a conducting direction with reference to the voltage occurring at the precision resistor. The precision resistor is dimensioned such that the voltage dropping off thereat is lower than the threshold voltage of the inverse diode of the field effect transistor.
The second electronic switch should likewise be inhibited during the entire inhibited phase of the first electronic switch. It can likewise be conductive during the entire turn-on phase of the first electronic switch. Since the peak value of the current measuring voltage to be stored only arises at the end of the conductive phase of the first electronic switch, the second electronic switch can potentially be driven so that it is switched on only in a temporal sub-region of the turn-on phase of the electronic switch lying in the main circuit, and is switched conductive such that both electronic switches simultaneously switch from the conductive into the inhibited condition. Turn-on current spikes when connecting the capacitor can be reduced in this way, or voltage spikes that appear at the precision resistor at the beginning of the current pulses flowing in the main circuit can be suppressed.
EP-A1-0 404 996 already discloses a clocked voltage converter that, in addition to comprising an electronic switch lying in the main circuit, comprises a further electronic switch that is arranged between a precision resistor lying in the main circuit and a RC parallel circuit. This additional electronic switch is switched on and off, and is delayed with reference to the former electronic switch. The voltage at the capacitor follows the test voltage at the precision resistor. The capacitor is already discharged at the point in time of the delayed turn-off of the additional electronic switch.
A run-out of the characteristic at which the limiting effect is reduced or canceled is a problem in clocked voltage converters. This can ultimately lead to the destruction of component parts. The cause of such a behavior lies in the remaining minimum pulse width of the turn-on pulse that results due to a series of dead times in the controlled chain. When a LC element having an inductor in the longitudinal arm and a capacitor in the shunt arm is arranged in the output circuit and--caused by said minimum pulse width, when the positive voltage/time area applied to the output inductor upon turn-on is higher than the negative voltage/time area in the turn-off phase--then the magnetic energy rises, and thus the current rises in unlimited fashion from period to period.
The so-called run-out of the characteristic upon occurrence of a short-circuit that is problematical in voltage converters having relatively high output voltage can be avoided with the assistance of the initially cited, known circuit arrangement since the output voltage of the device for peak value rectification controls the control circuit for the purpose of limiting the pulse current flowing in the main circuit.
Investigations within the framework of the invention, however, have shown that different control properties of the two electronic switches can have a negative influence in the efficiency of the desired current limitation. Due to component tolerances, the case can occur under certain circumstances that the second electronic switch, which is intended to effect the peak value rectification of the pulse-shaped current measuring voltage, is still in the conductive condition at a point in time at which the first electronic switch inhibits and the current measuring voltage thus drops to the value of zero.